Mobile satellite systems (or "MSSs") are satellite systems which support mobile voice and/or data services. MSSs have a number of advantages over more common terrestrial wireless communication systems such as cellular or personal communication systems (or "PCS"). One such advantage is that MSS systems typically provide larger coverage areas. MSS systems may also be more cost effective in undeveloped and lightly developed rural areas where a terrestrial mobile communication system may be both difficult and costly to install and maintain. For these reasons, MSS systems are often seen as the key to achieving global coverage for mobile communication systems.
MSS systems are not without their shortcomings, however. MSS systems may fail to deliver services when line-of-sight requirements are violated, for example, when service is attempted in urbanized areas populated with numerous high-rise buildings. Service from MSS systems is typically much more costly than service from terrestrial systems. MSS service also lacks many of the user features which have made cellular and PCS systems popular. Thus, from this perspective, cellular or PCS systems are often preferred over MSS systems when both are available.
While MSS and terrestrial wireless systems are often interconnected by way of the public switched telephone network (or "PSTN"), such interconnections have shortcomings. For example, in FIG. 1, an MSS system 10, representatively illustrated by a MSS satellite and a terrestrial wireless system, representatively illustrated by a mobile switching center 12 bi-directionally coupled to a base station 14, provide service coverage throughout geographic areas 16 and 18, respectively. The MSS system 10 is coupled to PSTN 26 by a gateway 28 (a satellite dish physically coupled to the PSTN 26 and in two-way radio communication with the MSS system 10) while the terrestrial wireless system 12 is coupled to the PSTN 26 by a local exchange carrier (or "LEC") 30 bi-directionally coupled to the MSC 12. Also shown in FIG. 1 are first, second and third mobile terminals 20, 22 and 24, all of which are located within the coverage area 16 of the MSS system 10 but only two of which (the mobile terminals 20 and 22) are located within the coverage area 18 of the terrestrial wireless system 12. If one assumes that the first mobile terminal 20 is configured for operation as an MSS system terminal while the second and third mobile terminals 22 and 24 are configured for operation as terrestrial wireless system terminals, a call originated by the first mobile terminal 20 and having the second mobile terminal 22 as its destination is directed from the first mobile terminal 20 to the MSS system 10, the gateway 28, the PSTN 26, the LEC 30, the MSC 12 and the base station 14 before arriving at the second mobile terminal 22. Thus, even though the first and second mobile terminals are in close physical proximity to each other, not only must a call between the two be directed along a circuitous route, but along a route subject to substantial toll charges. Furthermore, as the third mobile terminal 24 is outside the coverage area 18 of the terrestrial wireless system, a call from the first mobile terminal 20 to the third mobile terminal 24 cannot be completed even though the third mobile terminal 24 is within the coverage area 16 for the MSS system 10. Thusly, the existing interconnection between the MSS system 10 and the terrestrial wireless system 12 typically provides limited service at relatively high cost.
In order to ensure the broadest possible geographical coverage and to avoid unnecessarily expensive calls, a wireless subscriber must currently carry both terrestrial wireless and MSS phones. As such a solution is both cumbersome (because of the need to carry two phones and two phone numbers) and expensive (because of the need to contract with two service providers), a dual mode phone capable of operating within both MSS and terrestrial wireless systems with manual mode switching between the two systems has been proposed. However, such a phone lacks the ability to automatically change operating mode when a switch between systems would be advantageous to the user. Manual switching systems also require considerable user knowledge as to when a switch between satellite and terrestrial systems would be advantageous and lack the "seamless" switching preferred by many consumers for multi-mode terminals.
For example, for a mobile terminal to first switch from operation in a terrestrial wireless mode to operation in a MSS mode and to then register with a MSS system, the mobile terminal must overcome certain obstacles. More specifically, before the mobile terminal can initiate the registration process, the mobile terminal must first locate a MSS system, determine its access type and determine synchronization information such as differential and propagation delay factors. Similar obstacles make it difficult for a mobile terminal to switch from the MSS mode into the terrestrial wireless mode and then register with a terrestrial wireless system. While the mobile terminal can search for and acquire all of the information needed to initiate the registration process, the process can be quite time consuming. For example, much of the needed information can be obtained from the control channel for the MSS or terrestrial wireless systems broadcast. However, each system (or cell within a system) broadcasts at a different frequency within an operating band. Thus, to locate a control channel, the mobile terminal must scan the entire operating band. While the mobile terminal is searching for and acquiring information needed to register with a system operating in a different mode, it may be forced to suspend all other operations. Thus, the mobile terminal may lose calls and miss incoming pages, thereby reducing terminal availability and call delivery success rates.